import re

def is_pangram(sentence):
    '''Determine if a sentense str input is a pangram.
    
    Pangram means this sentense contains all 26 letters in alphabet (case insensitive).'''
    chrset = set(sentence)
    azset = set()
    for i in chrset:
        if re.match(r'[a-zA-Z]', i):
            azset.add(i.lower())
    if len(azset) == 26:
        return True
    return False
